#include<iostream>
using namespace std;

struct ListNode {
    int val;
    ListNode* next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode* next) : val(x), next(next) {}
    
};

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        return dfs(head);
    }

    ListNode* dfs(ListNode* head)
    {
        if (head == nullptr || head->next == nullptr)
        {
            return head;
        }

        ListNode* tmp = dfs(head->next->next);
        head->next->next = head;
        ListNode* ret = head->next;
        head->next = tmp;
        return ret;
    }
};

int main()
{
    return 0;
}
